The Federal Aviation Administration has set permissible noise level requirements for the area about major metropolitan airports, which must be met by jet aircraft. This is true not only of newly manufactured jet aircraft, but also of jet aircraft manufactured before the permissible noise levels were determined. Prior art workers have devoted considerable time and energy to the development of various types of noise abatement means to meet these requirements. At the present time, the approved noise suppression equipment includes special treatment of the inlet and exhaust portions of the engine nacelles by the airframe manufacturers, together with noise abatement means applied directly to the jet engines themselves. Special inlet and exhaust section treatments by the airframe manufacturers do not constitute a part of the present invention and preferably are used in conjunction with the teachings of the present invention.
The application of noise attentuation means directly to the jet engine, itself, has proven to be a relatively expensive treatment. While affective, such treatment of the jet engine, itself, does not produce as significant a noise reduction as does the inlet and exhaust treatments. When noise attenuation means are applied to the jet engines of existing aircraft, this requires removal of the engines from the aircraft, dismantling of the engines, replacement of the engines on the aircraft and recertification. This is both time consuming and expensive.
The present invention is based upon the discovery that noise reduction at least equivalent to that achievable by applying noise attentuation means directly to the jet engine can be achieved far more easily and less expensively by applying the noise suppression kit of the present invention to the interior surface of at least that portion of the engine nacelle surrounding the combuster chamber/turbine area of the jet engine. The kit comprises a plurality of individual noise attenuating modules affixable to and between the ribs or formers of the engine nacelle cowlings or doors. This can be easily accomplished by the airframe manufacturer during production of an aircraft. In addition, however, due to the modular nature of the noise suppression kit of the present invention, it can readily be applied to already existing aircraft by the airlines themselves, without the necessity of removing and dismantling the jet engines. As will be discussed hereinafter, the noise suppression kit of the present invention is flight worthy and does not present a flight or operational hazard.
While not intended to be so limited, for purposes of an exemplary showing the noise suppression kit of the present invention will be described in its application to DC-9-30 series aircraft manufactured by Douglas Aircraft Co. of Long Beach, Calif. and equipped with any JT8D -7, -7a, -7b, -9, -9a or -11 engine manufactured by Pratt and Whitney Aircraft of Hartford, Conn.